I added my account to my uncles so now we're doing family sharing, I recently got an Apple iTunes card for 15$ and can't use it because when I try to purchase something it goes to his credit card info to verify the purchase through him but I have credit so what's going. I'm getting fed up
You need to hit redeem in the App Store. If your account is logged in, then the credit will go to your account. And apple will use the credit first.